Yoghurt results from growth association of the two lactic acid bacteria, Streptococcus thermophilus and Lactobacillus bulgaricus. They grow together in milk where they ferment lactose, which is present at about 40-50 g per liter, to lactate. The whole fermentation usually takes 3 to 5 hours at 40.degree. C. and lowers pH from neutral to about 4.2. Upon storage at 4.degree.-12.degree. C. for several days, the pH then drops further to values below 4.0. Parallel to this increased acidity an appearance of bitterness develops and greatly degrades the organoleptic quality of the product.
It is L. bulgaricus which produces the typical yoghurt aroma during the fermentation process. Therefore, yoghurt missing or having a reduced titer of metabolic active L. bulgaricus cells miss the aroma components of the typical yoghurt. Thus, such yoghurt (or yoghurt like products) can be very neutral, i.e. flat in taste. This problem is particularly eminent for the production of mild or very mild yoghurt, which as a consequence of the limited growth rate of L. bulgaricus tend to lose a lot of their aroma.
The number of consumers that prefer the milder yet aromatic and flavorful yoghurt seems to have increased in recent years. Therefore, the control method of the acid production on the milk fermentation process and the preventing of the post-acidification, which is the pH dropping during the storage, were truly desired by yoghurt producers. Up to now, several methods or starter strain improvements have been suggested to control the acidification and post-acidification of yoghurt. Most of them are to reduce the number of the active cells in starter culture or to eliminate living cells from the finished product by pasteurizing. Alternatively, low acidifying strains are being used.
U.S. Pat. No. 4,734,361 (Murao et al.) describes such a Lactobacillus bulgaricus strain (OLL 1074) which has been deposited in Japan under the number FERM BP 1041.
This variant shows a weak tendency towards formation of lactic acid at lower temperatures.
By employing this variant, it is possible to produce a fermented milk or lactic acid beverage in which post-acidification at a lower temperature is reduced.
U.S. Pat. No. 5,071,763 (Somkuti et al.) describes that mutant strains of Streptococcus thermophilus having defective lactose transport systems and having a phenotype gluS, lacS-, sucS+ and .beta.gal+ are effective for use in processes where the hydrolysis of lactose is important. Thermostability of these strains as well as production of .beta.-gatactosidase allow lactose hydrolysis prior to and during pasteurization. These organisms provide the food industry with improved methods of making reduced lactose dairy products.
In L. bulgaricus, lactose is taken up by the lactose permease system and cleaved by the .beta.-galactosidase into its two moieties, glucose and galactose. Glucose is metabolized further to pyruvate, most of which is converted to lactate by the D-lactate dehydrogenase (D-LDH).
However, some of the pyruvate is decarboxylated to acetaldehyde, an important yoghurt aroma component, or channeled into other pathways resulting in aroma or aroma precursor elements.
Thomas et al. (J. of Bacteriology, May 1974, p. 329-333) and Smart et al. (Applied and Environmental Microbiology, March 1987, p. 533-541) describe that for Lactic streptococci, homolactic fermentation can be changed to heterolactic fermentation by reducing the LDH activity.
Payton et al. (FEMS Microbiology letters, 26, 1985, p. 333-336) describe that the LDH minus mutant of Bacillus stearothermophilus increase the amount of the ethanol production (following fermentation of glucose), whereas they lose the ability to produce lactate.
The document Journal of Bacteriology (vol. 144; no. 1, 1980, pages 217-221) shows that when the strain Lactobacillus bulgaricus NLS-4 grows anaerobically in continuous culture with limiting glucose, a shift in the pH of the medium from the acidic to the alkaline range causes this normally homofermentative bacterium to catabolize glucose in a heterofermentative fashion. The change in the fermentation is accompanied by a decrease in lactate dehydrogenase (LDH) biosynthesis in alkaline conditions.
However, yoghurt is an acid medium and this document does not suggest that it is possible to obtain a decrease in lactate dehydrogenase biosynthesis in acid conditons.